This application claims benefit of Japanese Patent Application No. 2000-378232 filed on Dec. 13, 2000, the contents of which are incorporated by the reference.
The present invention relates to variable wavelength optical transmitter, output control method therefor and optical communication system and, more particularly, to variable wavelength optical transmitter, output control method therefor and optical communication system, in which the output power level and wavelength of a variable wavelength semiconductor laser element is controlled.
Owing to explosive spread of internet systems, there is an increasing demand for transmission capacity increase. This means that not only optical transmission using wavelength of a single semiconductor laser diode but also DWDM (Dense Wavelength Division Multiplex) communication with dense multiplexing of a plurality of wavelengths are becoming more and more important. In the DWDM communication, reduction of the oscillation wavelength interval of the semiconductor laser diode from 100 GHz to 50 GHz or 25 GHz has been demanded. In addition, the number of necessary wavelengths is increasing from 80 to 160, 320 and 640, thus dictating semiconductor laser diodes capable of providing pluralities of wavelengths.
Heretofore, some methods of maintaining extremely narrow wavelength interval, have been proposed as techniques for ensuring semiconductor laser wavelength stability. As an example, according to Japanese Patent Laid-Open No. 10-209546, a forward light beam emitted from a semiconductor laser diode is split by an optical coupler disposed outside a package, and the transmitted and reflected light beams are filtered through interference optical filters and detected by optical detectors, respectively. The oscillation wavelength of the semiconductor laser diode is controlled by using the difference between the two detected light quantities.
To the same end, there are also methods, in which the rearward light beam from a semiconductor laser diode is utilized. In these methods, usually the rearward light beam from a semiconductor laser diode is detected by an optical detector for constant semiconductor laser diode output light beam level control. As for oscillation wavelength variations, such arrangements as for constant temperature control are utilized for suppressing the wavelength variations.
An example of such arrangement is disclosed in Japanese Patent Laid-Open No. 9-219554. In this technique, the temperature of a semiconductor laser diode is controlled for oscillation wavelength control by using two optical filters having transmission characteristics with opposite slopes.
FIG. 14 shows the prior art construction of a semiconductor laser diode wavelength control device disclosed in the Japanese Patent Laid-Open No. 9-219554. In this device, the rearward light beam emitted from a semiconductor laser diode 22 is split by a beam splitter 23 disposed therebehind, and the resultant transmitted and reflected light beams are filtered through wavelength filters 24 and 25 having opposite transmission characteristics and detected by light beam level detectors 26 and 27, respectively.
The light beam detectors 26 and 27 output light beam level detection signals, which are processed in control circuits 28 and 29, whereby the output light beam level and the oscillation wavelength of the semiconductor laser diode 22 are controlled by an LD drive circuit 30 and a temperature control circuit 31. In this construction, the oscillation wavelength of the semiconductor laser diode is controlled by controlling the temperature thereof such as to compensate for the oscillation wavelength deviation based on the difference between the light beam levels detected by the light beam level detectors 26 and 27.
In the above DWDM communication system, the wavelength interval has been shifted from 100 GHz to 50 GHz, and as its light beam transmitter is used, for instance, a variable wavelength light beam source element as shown in FIG. 4. Usually, the wavelength of the semiconductor changes with 10° C. thereof by about 1 nm. With this variable wavelength light beam source element used for the DWDM with wavelength interval of 50 GHz, it is possible to cover five channels (at the time of the 50 GHz wavelength interval) by temperature control of one semiconductor laser diode by ±10° C. Thus, as shown in FIG. 5, by arraying four semiconductor lasers at an interval of 2 nm, it is possible to cover 20 channels with the four semiconductor lasers.
In the prior art disclosed in the above Japanese Patent Laid-Open No. 10-209546 or Japanese Patent Laid-Open No. 9-219544, where the variable wavelength light beam source element having the above construction is used for wavelength stabilization control of semiconductor lasers 351 to 354 constituting the semiconductor laser array shown in FIG. 4, the detectable wavelength range of the wavelength versus permeability characteristic as a characteristic of a wavelength deviation sensor for detecting the wavelength deviation, is narrow, and it is therefore impossible to detect wavelength deviation (i.e., wavelength variation) from each of the wavelengths of a plurality of channels.
Also, the application of the above Japanese Patent Laid-Open No. 10-209546 to a DWDM wavelength stabilization system, poses a problem of giving rise to the optical transmission device size increase in that it is necessary to split the forward light beam with an expensive polarization maintaining coupler for reducing adverse effects of the polarization and also in that when the number of multiplex wavelengths is increased due to necessity of externally using the semiconductor laser module, it is necessary to add wavelength deviation detecting mechanisms corresponding in number to the number of wavelengths, thus requiring an increased space of mounting.
With the construction disclosed in the above Japanese Patent Laid-Open No. 9-219554, it is possible to provide some measure for oscillation wavelength stabilization of the semiconductor laser diode against the secular deterioration thereof. However, to obtain a sufficient received light beam current quantity for control such that the rearward light beam is inputted without being collimated to the optical detector, the optical detector should be disposed in the proximity of the semiconductor laser diode, and it is thus difficult to dispose the filter between these two components. In the variable wavelength light beam source element comprising monolithic integration of a semiconductor laser array, a synthesis unit and a light beam amplifier region is disclosed in Japanese Patent Laid-Open No. 2000-012952, due to the semiconductor laser array it is impossible to receive the rearward light beam uniformly by using a photo-diode. Besides, since the output light beam is adjusted in the light beam or the like by monitoring the forward light beam.
Morover, like the case of the above Japanese Patent Laid-Open No. 10-209546 technique, with an increased number of multiplex wavelengths it is necessary to provide wavelength deviation detecting mechanism corresponding in number to the number of the wavelengths, thus requiring an increased space of mounting and giving rise to the optical transmission device size increase.